High temperature friction behavior of CrVxN coatings

The object of this research is to deposit CrVxN coatings with different content of Vanadium (V) by ion-sputter deposition to gain fundamental knowledge on the friction behavior of these coatings under high temperature and to analyze stick-slip phenomenon. The results of stick-slip tests under low sliding velocities are compared with real contact conditions. Here, an entirely new method is established to approach this problem at different scales by combining high-temperature friction study of coatings (Fraunhofer Institute for mechanics of materials, IWM), and micro-tribology for stick-slip analysis (Holon Institute of Technology, HIT). The tribological experiments at Fraunhofer IWM were performed using an oscillating friction-wear test rig (SRV4, Optimol Instruments) in the temperature range of 25 to 700 °C. To study stick-slip phenomena, a new ball on flat device with a heated chamber for experiments at up to 500 °C was developed at the HIT. It was shown that an addition of V up to 27-35 at.% in CrVxN coatings increases the hardness, fracture toughness and decreases the grain size. These parameters correlate with improved tribological properties of coatings with high content of V. At room temperature CrVxN coatings feature strong sticking effects in the friction experiments. High values of the stick-slip parameters are explained by mechanical interlocking and adhesion on contact spots due to softening of surface layers of balls, high roughness and transfer of thick iron oxide films on the surface of the coatings. Low values of the wear and stick-slip parameters under friction of Cr0.65V0.35N at high temperatures (T = 500-700 °C) is associated with the formation of V2O5 oxide phase providing easy shearing of tribofilms in the interface of rubbed surfaces.